As confidence in the plutonium bomb design grew it was decided not to use Jumbo. Instead, it was placed in a steel tower about 800 yards from ground zero. The blast destroyed the tower, but Jumbo survived intact.
Today Jumbo rests at the entrance to ground zero so all can see it. The ends are missing because, in 1946, the Army detonated eight 500-pound bombs inside it. Because Jumbo was standing on end, the bombs were stacked in the bottom and the asymmetry of the explosion blew the ends off.
To calibrate the instruments which would be measuring the atomic explosion and to practice a countdown, the Manhattan scientists ran a simulated blast on May 7. They stacked 100 tons of TNT onto a 20-foot wooden platform just southeast of ground zero. Louis Hemplemann inserted a small amount of radioactive material from Hanford into tubes running through the stack of crates. The scientists hoped to get a feel for how the radiation might spread in the real test by analyzing this test. The explosion destroyed the platform, leaving a small crater with trace amounts of radiation in it.
Bomb Assembly
On July 12 the two hemispheres of plutonium were carried to the George McDonald ranch house just two miles from ground zero. At the house, Brig. Gen. Thomas Farrell, deputy to Maj. Gen. Leslie Groves, was asked to sign a receipt for the plutonium. Farrell later said, "I recall that I asked them if I was going to sign for it shouldn't I take it and handle it. So I took this heavy ball in my hand and I felt it growing warm, I got a certain sense of its hidden power. It wasn't a cold piece of metal, but it was really a piece of metal that seemed to be working inside. Then maybe for the first time I began to believe some of the fantastic tales the scientists had told about this nuclear power."
At the McDonald ranch house the master bedroom had been turned into a clean room for the assembly of the bomb core. According to Robert Bacher, a member of the assembly team, they tried to use only tools and materials from a special kit. Several of these kits existed and some were already on their way to Tinian, the island in the Pacific which was the base for the bombers. The idea was to test the procedures and tools at Trinity as well as the bomb itself.
At one minute past midnight on Friday, July 13, the explosive assembly left Los Alamos for Trinity Site. Later in the morning, assembly of the plutonium core began. According to Raemer Schreiber, Robert Bacher was the advisor and Marshall Holloway and Philip Morrison had overall responsibility. Louis Slotin, Boyce McDaniel and Cyril Smith were responsible for the mechanical assembly in the ranch house. Later Holloway was responsible for the mechanical assembly at the tower.
In the afternoon of the 13th the core was taken to ground zero for insertion into the bomb mechanism.
The bomb was assembled under the tower on July 13. The plutonium core was inserted into the device with some difficulty. On the first try it stuck. After letting the temperatures of the plutonium and casing equalize the core slid smoothly into place. Once the assembly was complete many of the men took a welcome relief and went swimming in the water tank east of the McDonald ranch house.
The next morning the entire bomb was raised to the top of the 100 foot steel tower and placed in a small shelter. A crew then attached all the detonators and by 5 p.m. it was complete.